Preheat the oven to 180˚C fan / 200˚C / 356˚F convection / 392˚F.
Line a 20 cm x 20cm (8" x 8") square baking tin with baking paper / baking parchment.
Melt the butter in a small saucepan over a gentle heat. Once the butter has melted take off the heat and leave to cool.
Optional: If you want to make brown butter blondies, continue to cook the butter for another 5-7 minutes until the foam subsides, the butter smells nutty, and the solids at the bottom of the pan are a golden brown. Tip the butter in to a bowl to stop it from cooking (it can easily burn if left in the pan).
Tip the brown and white sugar to a medium to large mixing bowl and pour in the cooled melted butter and using an electric hand whisk, whisk until just combined.
Add the eggs, vanilla extract and lemon zest and whisk until light and fluffy and doubled in size.
Sift in the flour, baking powder and salt, and gently stir with a spoon until just combined.
Fold in the chopped white chocolate and raspberries.
Spoon the mixture in to the prepared tin and flatten out gently with a spoon.
Place in the preheated oven for 25-28 minutes, until a skewer inserted into the middle of the blondies comes out sticky, but not covered in raw batter.
Cool the blondies in the tin for 15 minutes then carefully lift out of the tin and place on a wire rack to cool completely.
If you want to drizzle with chocolate, melt the white chocolate in a small ban marie (a small heatproof bowl set over a small saucepan of barely simmering water, making sure the water doesn't touch the bowl). Or melt in the microwave and then drizzle over the blondie.
Cut in to 16 squares.
Store in an airtight container at room temperature for up to 3-4 days, or freeze for up to 1 month.